Well, the truth is that in this case we wouldn't go slang and we would say: «Είμαι εντελώς άσχετος στα βιντεοπαιχνίδια» and «Δεν ξέρω καθόλου μπουζούκι». If we want to tell someone that he really sucks at these two, we could ask him: «Κουλός είσαι;»